1. Field of the Invention
The present invention relates generally to cargo transfer cranes and particularly to a hoisting crane for loading and unloading cargo ships.
2. Description of the Prior Art
It is well known that the cost of lay days for a ship in harbor for on loading or off loading can prove particularly expensive to a ship's owner. Consequently, various methods have been developed for expediting the on loading and off loading of ships. One such method contemplates the loading of pieces of cargo in large containers to be contained therein for transfer to and from the ship as a unit. These containers are then hoisted directly from the trailer at dock side by a gantry crane which then transfers them to the ship's hold.
The rate of compensation which must be made to longshoreman personnel and truckers providing the labor associated with maritime cargo transfer is such as to demand the utmost efficiency in achieving such transfer. Because of the inordinate weight associated with the loaded cargo containers, safety is of extreme importance in avoiding dislocation thereof which may cause damage to the cargo and, more importantly, personal injury to the dock workers.
Truck drivers are often paid on a flat rate per delivery basis thus providing considerable incentive for rapid unloading of cargo from trailers towed by their truck tractors. The containers are typically loaded on the truck trailers and locked thereto to prevent shifting of the containers relative to the trailer during transport to the dock site. A recognized problem is the fact that the truckers or other responsible personnel, from time to time, neglect to unlock such locking mechanism thus leaving the container locked to the truck as the crane hoisting mechanism raises the container. This oftentimes results in lifting of the truck itself and sometimes the attached truck tractor with the driver aboard. Then, when the locking mechanism fails, or otherwise releases, freeing the trailer from the container, the trailer and truck tractor may be dropped on to the underlying dock from a considerable height, such as 20 or more feet above the dock, often resulting in damage to the truck and trailer and injury to the driver. Consequently, there exists a need for a safety apparatus which will limit hoisting of such containers when locked to the trailer.
Detection systems have been proposed in transfer cranes for detecting and controlling the distance between multiple trolleys mounted on such crane. A device of this type is shown in U.S. Pat. No. 4,753,357 to Miyoshi. While satisfactory for their intended purpose, such devices fail to detect an unsafe condition in a cargo truck transporting cargo to the crane.
Other mechanisms have been proposed for detecting the attitude of a crane to indicate an unsafe condition. A device of this type is shown in U.S. Pat. No. 4,743,893 to Gentile et al.
Still further devices have been proposed for detecting potential collision between material handling devices and to protect against such collisions. A device of this type is shown in U.S. Pat. No. 2,814,032 to Agnew et al. Crane safety cut off and overload devices have been proposed to enhance the safety of the operator and attendant personnel. Devices of this type are shown generally in U.S. Pat. Nos. 4,804,095 to Rohr et al., 4,003,482 to Cheze, 4,787,524 to Cobb et al. and 5,058,752 to Wacht et al., as well as U.S. Pat. No. 4,821,835 to Latvys et al. and 3,824,578 to Harders. However, none of these devices detect an unsafe condition in a cargo transport truck or respond to such a condition to render a loading crane inoperative.
In our co-pending patent application, now U.S. Pat. No. 5,260,688, the content of which is incorporated herein by reference, we disclosed the broad concept of a crane control system responsive to an unsafe condition in any one of a number of trucks serving such crane to render it inoperative. While that system has gained considerable acceptance, it has the shortcoming that under certain conditions the truck mounted sensors could be activated by a false signal such as in the instance when the sensing wand on the truck is accidentally or intentionally struck thus generating a false signal. In addition, the truck driver when delivering to a gantry crane must select the proper transmitter corresponding to the respective gantry crane which, if not properly selected, could disable the safety device.